1. Field of Invention
The invention relates to a laser drive control apparatus, especially for a multibeam scanner comprising a plurality of beam emitting points.
2. Description of Related Art
Conventional multibeam scanners emit a plurality of laser beams from laser diodes simultaneously and scan a scan surface of a photosensitive material (such as a photosensitive drum) with the plurality of laser beams, thereby forming an image on the scan surface. Multibeam scanners scan a surface at a higher speed than single-laser scanners, and thus improve recording speed.
To keep high image quality, it is necessary for multibeam scanners to perform Automatic Power Control (APC), because the respective laser diodes of the scanners have difficulties in stabilizing the beam amount of the laser beam therefrom. Even if each laser diode is driven with a constant driving current, the temperature of the laser diode increases due to its beam emission, and the luminous efficiency of the laser diode decreases with the temperature increase.
Therefore, as disclosed in Japanese Patent No. 63-42432, multibeam scanners perform APC for feeding a driving current to the laser diode, so that the laser diode always emits the laser beam of an appropriate beam amount. More specifically, the Japanese Patent No. 63-42432 discloses a multibeam scanner having a control circuit, a constant-current circuit, and a photoreceptor. The control circuit controls each of the laser diodes that emit a laser beam. The photoreceptor detects the laser beams emitted from the laser diodes and determines beam amounts. The photoreceptor gives a feedback of the beam amounts to the control circuit. Upon reception of the feedback, the control circuit compares the feedback with reference values. Based on the comparison, the control circuit controls the constant-current circuit to feed the appropriate driving currents so that the respective laser diodes emit the laser beams of appropriate beam amounts. The reference values are provided for each of the respective laser diodes.
However, all the laser diodes do not always have the same current-emission characteristic. Even if the laser diodes have the same current-emission characteristic, it is inevitable that incident angles against the photoreceptor vary from laser beam to laser beam. Such variations in incident angles are caused by the laser diodes not being properly placed in exact positions and orientations, or by broadening of the laser beams. Therefore, in order to perform APC properly, the respective reference values for the laser diodes need to be provided.
Conventional multibeam scanners comprise a plurality of output circuits for outputting the reference values, respectively, for the laser beams. Further, it is necessary to adjust the setting of each of the output circuits on an individual basis, so that each of the output circuits outputs the appropriate reference value. Thus, such conventional multibeam scanners are expensive, time consuming and labor consuming to manufacture.
In the view of the foregoing, the present invention has been developed to resolve the above-mentioned and other problems.
According to one aspect of the invention, there is provided a laser drive control apparatus for controlling a plurality of beam emitting points to emit a plurality of laser beams, respectively, to form scanning lines during a predetermined time period, the laser drive control apparatus comprising a first control unit that controls a plurality of beam emitting points to emit a plurality of laser beams successively except during a predetermined time period, a photoreceptor that detects each of the laser beams emitted under the control of the first control unit and generates a beam amount signal indicating a beam amount of the laser beam for each of the plurality of beam emitting points, a providing unit that provides a plurality of reference values corresponding to the plurality of beam emitting points, respectively, when the photoreceptor generates the beam amount signals, a comparison unit that compares the beam amount signals with the corresponding reference values, respectively, and a second control unit that controls, based on the comparison made by the comparison unit, the plurality of beam emitting points to emit the laser beams to form the scanning lines during the predetermined time period, and wherein the plurality of reference values are set so that the total sum of the reference values is a predetermined value.
According to another aspect of the invention, there is provided a method for controlling a plurality of beam emitting points to emit a plurality of laser beams, respectively, to form scanning lines during a predetermined time period, comprising first controlling a plurality of beam emitting points to emit a plurality of laser beams successively except during a predetermined time period, detecting each of the laser beams emitted in the first controlling step, generating a beam amount signal indicating a beam amount of the laser beam for each of the plurality of beam emitting points, providing a plurality of reference values corresponding to the plurality of beam emitting points, respectively, when the beam amount signals are generated, comparing the beam amount signals with the corresponding reference values, respectively, and second controlling, the plurality of beam emitting points based on the comparison made in the comparing step to emit the laser beams to form the scanning lines during the predetermined time period, and wherein the plurality of reference values are set so that the total sum of the reference values is a predetermined value.
According to still another aspect of the invention, there is provided a method of setting a plurality of reference values corresponding to a plurality of beam emitting points, respectively, for use in controlling the plurality of beam emitting points to emit laser beams of appropriate amounts, the method comprising the step of balancing the plurality of reference values so that the total sum of the plurality of reference values is a predetermined value.